Conventional variable focal length lens systems, for example, zoom lens systems, typically include at least one group of lenses that is moved along an optical axis in order to change the focal length of the system resulting in variable magnification of an object at an image plane of the system. Zoom lens systems can include multiple moving groups or a combination of fixed lens groups and moving lens groups. These types of lens systems have various design forms with each design form having its own set of strengths in terms of size, performance, cost, mechanical complexity, etc. Conventional variable focal length lens systems mechanically move lens groups to achieve variable focal length. This movement adds cost, complexity, and size to the lens system. In consumer camera applications, for example, it is desirable to have a compact and inexpensive zoom lens system.
Variable power lens elements are known. For example, U.S. Pat. No. 6,369,954 B1 and PCT Application Publication No. WO 2003/069380 disclose electrically controllable liquid lenses. Each of these lenses is composed of two immiscible liquids having different refractive indices contained in a cavity having transparent outer surfaces. Electrodes are located on inner sides of the cavity. When a voltage is applied to the electrodes, the curvature of the interface between the two liquids changes in response to an electric field that is established between the electrodes thereby causing a change in the optical power of the liquid lens. The response time of the electrically controllable liquid lenses is approximately 0.02 to 0.1 seconds and the size is typically 2 to 10 mm. The shape of the optical surface of these liquid lenses is minimally adjustable through modification of the electrodes allowing astigmatism to be present in the optical system.
U.S. Pat. No. 6,369,954 B1 discloses the use of concave or convex cavity surfaces to obtain a particular diopter value of the device at rest and first and second liquids being non-miscible, of different optical indexes, and of substantially same density with the first liquid being conductive and the second liquid being insulating. Additionally, the conductive liquid may be made conductive by the addition of ionic components. PCT Application Publication No. WO 2003/069380 discloses the use of liquids having a refractive index between 1.25 and 1.60 with added molecular constituents to increase the density of one of the liquids to enable the densities of the two liquids to be matched thereby avoiding gravitational effects. Additionally, the transparent outer surfaces have aspheric shapes to provide desired initial focusing characteristics and to act as a field flattener.
US Patent Application Publication Nos. US 2004/0179280 and US 2005/0018127 disclose liquid crystal lenses. In this type of lens, a liquid crystal material is held between a series of electrodes. Applying a voltage to the electrodes causes the liquid crystals to align with the electric field such that the optical power of the liquid crystal lens changes. Liquid crystal lenses response time can be as little as 0.003 seconds with a size range similar to electrically controllable liquid lenses. As with the liquid lenses, the electric field determines the overall optical quality of the liquid crystal lens. As such, liquid crystal lenses have minimal adjustability through electrode modification and astigmatism is present in the lens system.
U.S. Pat. No. 5,574,598 discloses pressure controllable liquid lenses. This lens includes a liquid filled cavity having transparent surfaces made of an elastomeric material. When pressure is applied to an adjacent cavity, the elastomeric material surfaces of the liquid filled cavity can be stretched into a curved shape. When this happens, the liquid lens has optical power. The optical power of the lens can be controlled within the elastic limits of the transparent elastomeric material by varying the pressure applied to the adjacent cavity. In pressure controllable liquid lenses, the optical surface tends to be spherical in shape since the surface is determined by surface tension. Consequently, astigmatism is also present in this lens assembly.
In the article “Variable-focus lens with 1-kHz bandwidth”, by H. Oku, K. Hashimoto, M. Ishikawa, Optics Express, Vol. 12, No. 10, May, 2004, pages 2138-2149, a pressure controllable liquid lens is described. This lens also includes a liquid filled cavity having transparent surfaces made of an elastomeric material. A piezoelectric actuator is used to apply a pressure to the transparent surfaces in order to change the shape of these surfaces. Varying the voltage applied to the piezoelectric actuator varies the amount of pressure applied to the lens. This varies the optical power of the lens. The speed of response of this lens system is approximately 0.001 seconds. Astigmatism is also present in this pressure controllable liquid lens.
While any of the lenses described above can be used in variable focal length lens systems, the response time (under 0.1 seconds) and size (typically 2 to 10 mm) characteristics of electrically controllable liquid lenses and liquid crystal lenses make these lenses particularly suitable for use in zoom lenses for consumer cameras. Pressure controllable lenses can also be used provided the size of the lens is compatible to the application contemplated.
Zoom lenses including an electrically controllable variable power lens are known. For example, PCT Application Publication No. WO 2004/038480 discloses a zoom lens comprising, from the object side to the image side, a front fixed lens group, a controllable lens group, and a rear fixed lens group. The controllable lens group comprises a voltage-controlled electrowetting device which contains a first fluid and a second fluid having different refractive indices in the range 1.25 to 1.60 with at least two first fluid-second fluid interfaces. The curvatures, and therefore the lens power, of these interfaces can be changed independently by supplying a voltage to electrodes of the device, so that no mechanical movement of lens elements is needed.
PCT Application Publication No. WO 2004/083899 discloses an optical component for changing angular magnification of an imaging device. The optical component comprises a chamber having a first substrate on one side and a second substrate on the opposite side for separately disposing a first liquid drop and a second liquid drop along an optical axis. The chamber is also filled with a liquid which is different from the first and second liquid drops. The optical component also has a first electrode adjacent to the first side, a second electrode adjacent to the second side, and a third electrode layer between the first and second electrode layers for applying different electric fields on the first and second liquid drops to change the focal lengths of the first and second liquid drops, without changing the sum of the focal lengths.
The zoom lenses described above have limitations. Each of the variable power lenses described in these lens systems provides multiple variable optical surfaces that tend to induce an astigmatism component into each lens system when voltage is applied to the variable power lens. The astigmatism produced by the use of variable power lenses should be corrected or at least reduced in order to improve the image quality of these zoom lenses. While there is also a desired to reduce the number of moving groups in zoom lenses in order to reduce the overall size and complexity of the lenses, eliminating all the moving groups increases astigmatism in the lens system and sacrifices imaging quality. Additionally, and at least in the case of the liquid lens systems described above, the various competing requirements for the liquids, for example, density, refractive index, freezing point, boiling point, conductivity, etc., make selection of the appropriate liquid very difficult to optimize.
As such, there is a need for variable focal length lens systems having a reduced number of moving lens groups, and less adverse effects caused by aberrations, such as astigmatism, lateral color, etc. There is also a need for independent adjustment of at least one of the characteristics of the liquids used in liquid lenses. These characteristics include, for example, density, refractive index, freezing point, boiling point, conductivity, etc.